Variable displacement refrigerant compressors have been utilized in automotive air conditioning systems, with the displacement regulated in accordance with cooling demand via either a hydraulic control valve or solenoid control valve. In a typical arrangement, the compressor includes one or more pistons coupled to a tiltable wobble plate or swash plate, and the control valve adjusts a differential pressure acting on a wobble plate control mechanism to vary the wobble plate tilt angle, and hence the compressor displacement or stroke.
Various sensing devices have been proposed for determining the compressor speed and stroke, either for control or diagnostic purposes. In general, the sensing devices include a magnet mounted on a reciprocating element of the compressor, and a magnetic sensor mounted in or on the compressor housing in proximity to the reciprocating magnet. As the magnet reciprocates, the sensor develops a pulse or quasi-sinusoidal voltage waveform. The frequency of the waveform is typically independent of compressor stroke, and can be used as a measure of compressor speed, whereas the duty cycle of the waveform varies with the angle of the wobble or swash plate, and can be used as a measure of compressor stroke. Generally speaking, these devices are problematic because (1) they require changes in the mechanical design of the compressor, (2) the stroke measurement is non-linearly related to the actual stroke, and (3) the duty cycle measurements are subject to significant variation due to noise in the sensor output signal. As a result, a sensing system based on the known techniques is both costly and unreliable.